The present invention generally relates to optical switching and, more particularly, to techniques for implementing broadband optical switching arrangements having very low crosstalk.
There is a need to minimize the crosstalk in large Nxc3x97N optical space switches realized using 1xc3x972 or 2xc3x972 switching elements. When N is large, the total crosstalk contribution from the various elements becomes correspondingly large. Typically, the crosstalk performance of an 2xc3x972 element realized using conventional techniques is unsatisfactory in several respects. Ideally one would like crosstalk to be negligible over a wide wavelength range. Instead, for a conventional Mach-Zehnder arrangement, negligible crosstalk over a wide wavelength range is only possible when the element is in the cross state (the input signals are interchanged, i.e., input 1 goes to output 2, etc.). For the bar state (the input signals are not interchanged, i.e., input 1 goes to output 1, etc.) negligible crosstalk is only possible in the vicinity of a particular design wavelength xcex0. Moreover, for the bar state, zero crosstalk transmission at xcex0 strictly requires precise control of the coupling coefficients of the input and output couplers connected to the arms of the Mach-Zehnder. This condition is difficult to realize in practice, because of fabrication errors. Thus the coupling coefficients typically deviate appreciable from the specified values, and appreciable crosstalk is produced in the bar state.
Thus, there is a continuing need to minimize crosstalk in large Nxc3x97N space switches, implemented using 1xc3x972 and 2xc3x972 switching elements, over a wide range of optical wavelengths.
In accordance with the switching apparatus and operating method of the present invention, I have significantly reduced crosstalk by designing 1xc3x972 and 2xc3x972 switching elements using an imaging arrangement of three arms combined with two star couplers. In one embodiment of a 1xc3x972 switching element, the input star coupler is replaced with an optical signal splitter. Application of the 1xc3x972 and 2xc3x972 switching elements of the present invention include the realization of a dilated Nxc3x97N broadband switch with negligible crosstalk over a very wide wavelength stopband characterized by at least two equally spaced zeros. The very wide crosstalk stopband enables the Nxc3x97N broadband switch to switch a broadband wavelength division multiplexed input signal to one particular output port with very low crosstalk at the other output ports.
More particularly, in accordance with the present invention, my optical switching apparatus comprises
an imaging arrangement including a first and second couplers having three imaging waveguide arms connected therebetween;
the first coupler including at least one input connected as an input waveguide of the switching apparatus and three outputs connected to the three imaging arms;
the second coupler is a star coupler consisting of a first and second radial array separated by a slab waveguide, the three imaging arms being connected to three central waveguides of the first radial array, and two central waveguides of the second radial array being connected to two output waveguides of the switching apparatus;
the three imaging arms including a top, a central, and a bottom imaging arm;
at least two of the three imaging arms including wavelength adjusters to control optical path lengths through the three arms, said adjusters adjusted to produce equal optical path lengths from the input waveguide to a first output waveguide thereby maximizing power transfer from the input waveguide to the first output waveguide; and
the spacing between the two central waveguides of the second radial array being selected so as to minimize power transfer to the second waveguide; and
wherein the difference between the combined power transferred to the top and bottom imaging arm and the power transferred to the central imaging arm is within a predetermined value.
My operating method is applied to an optical switching apparatus comprising
an imaging arrangement including a first and second couplers having three imaging waveguide arms connected therebetween;
the first coupler including at least one input connected as an input waveguide of the switching apparatus and three outputs connected to the three imaging arms;
the second coupler is a star coupler consisting of a first and second radial array separated by a slab waveguide, the three imaging arms being connected to three central waveguides of the first radial array, and two central waveguides of the second radial array being connected to two output waveguides of the switching apparatus; and
the three imaging arms including a top, a central, and a bottom imaging arm;
my method comprises the steps of:
adjusting the phase of a optical signal in at least two of the three imaging arms to produce equal optical signal phase delay from the input waveguide to a output waveguide thereby maximizing power transfer from the input waveguide to the first output waveguide; and
wherein the difference between the combined power transferred to the top and bottom imaging arm and the power transferred to the central imaging arm is within a predetermined maximum value.